Process for producing isobutylene and methanol

ABSTRACT

A process for producing isobutylene and methanol, comprising decomposing methyl-tert-butylether into isobutylene and methanol, and separating into isobutylene and methanol, thereby individually recovering isobutylene and methanol, the process comprising the following steps: 
     a first step of subjecting methyl-tert-butylether to decomposition in the presence of a solid acid catalyst; 
     a second step of washing the resultant with water to separate into an oil layer and a water layer; 
     a third step of subjecting the oil layer to distillation to obtain a fraction from the top of the distillation column and a fraction from the bottom thereof; and 
     a fourth step of subjecting the water layer to distillation to obtain a fraction containing methanol from the top of the distillation column, a fraction containing water from the bottom thereof and a fraction rich in tert-butanol from a side cut thereof, and then recycling a part or whole of the fraction from the side cut to the first step.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a process for producing isobutylene andmethanol. Particularly, the present invention relates to a process forproducing isobutylene and methanol, in which methyl-tert-butyl ether isdecomposed into isobutylene and methanol, isobutylene and methanol thusobtained are separated respectively thereby recovering isobutylene andmethanol individually, the process being capable of suppressingcontamination of methanol as an objective product, with water andtert-butanol and preventing losses of methanol and isobutylene.

2. Description of Related Arts

Methods of recovering isobutylene and methanol by decomposingmethyl-tert-butyl ether into isobutylene and methanol and separatingisobutylene and methanol respectively therefrom are known (e.g.JP47-041882B).

Further, U.S. Pat. No. 4,544,776 discloses that, in the case ofetherifying C₄-C₇ iso-olefins with methanol, the etherificationproduct(for example, methyl-tert-butyl ether) containing alcoholcomponents such as methanol and a tertiary alcohol (e.g. tert-butanol)is extracted with an excess of water utilizing a washing column directlyconnected to the reactor as an aqueous extract; when the aqueous extractis distilled utilizing a distillation column, a tert-butanol-waterbinary azeotrope is taken overhead in this column and thus undesirablyincreases the water content in methanol to be recover; and when thismethanol is reused for etherification, the water contained increases theformation of tert-butanol and thus the increase ultimately results in anadditional “snowball effect”. As a solution to the above problem, thepatent discloses a process wherein an alcohol component is extractedusing an excessive amount of water so that a ratio of methanol/water tobe extracted becomes 1:2 to 1:5, then the aqueous extract thus obtainedis distilled to recover methanol from the top of the distillation columnand a highly concentrated tert-alcohol is drawn as a side-stream to beretuned to the washing tower. However, the above process isnon-economical since the ratio of water is excessively high, and theprocess has a drawback that tert-butanol formed by the etherificationreaction is ultimately contained in product methyl-tert-butyl ether asone of impurities.

In addition, in a process of recovering isobutylene and methanol bydecomposing methyl-tert-butyl ether into isobutylene and methanol andseparating isobutylene and methanol respectively therefrom, tert-butanolis also usually contained in methyl-tert-butyl ether as a raw material,in an amount of 0.2 to 0.8 wt %. Depending on a decomposition ratio ofmethyl-tert-butyl ether, though 50 to 80% of tert-butanol is usuallydecomposed into isobutylene and water in the first step, undecomposedtert-butanol is distilled from the top of the distillation column in themethanol recovery step due to the residual tert-butanol, to therebycause a problem that a water content in methanol is increased due to theazeotropy of tert-butanol and water.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producingisobutylene and methanol, wherein isobutylene and methanol are recoveredby decomposing methyl-tert-butyl ether into isobutylene and methanol toobtain a mixture thereof, and separating isobutylene and methanoltherefrom, the process being capable of suppressing a contamination ofmethanol as an objective product with both of water and tert-butanol andpreventing from losses of isobutylene and methanol.

Namely, the present invention relates to a process for producingisobutylene and methanol comprising decomposing methyl-tert-butyl etherinto isobutylene and methanol to obtain a mixture containing isobutyleneand methanol, separating isobutylene and methanol respectivelytherefrom, thereby individually recovering isobutylene and methanol, theprocess comprising the following steps:

a first step of subjecting methyl-tert-butyl ether to decomposition inthe presence of a solid acid catalyst to obtain a reaction liquidcontaining isobutylene, methanol, and, as by-products, a dialkyl etherand heavy components containing an isobutylene dimer and an isobutylenetrimer;

a second step of washing the reaction liquid obtained in the first stepwith water to separate the reaction liquid into an oil layer and a waterlayer;

a third step of subjecting the oil layer obtained in the second step todistillation using a distillation column to obtain a fraction containingisobutylene and the dialkylether from the top of the distillation columnand a fraction containing methyl-tert-butyl ether and the heavycomponents from the bottom of the distillation column; and

a fourth step of subjecting the water layer obtained in the second stepto distillation using a distillation column to obtain a fractioncontaining methanol from the top of the distillation column, a fractioncontaining water from the bottom of the distillation column and afraction rich in tert-butanol from a side cut of the distillationcolumn, and then recycling at least a part of the fraction obtained fromthe side cut to the first step.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a flow of an example of a production process according tothe present invention.

EXPLANATION OF SYMBOLS

1. First step, 2. Second step, 3. Third step, 4. Fourth step, 5. Methyltertiary butyl ether (raw material for decomposition), 6. Methanol(objective product), 7. Isobutylene (objective product), 8. Side cut, 9.Freshwater, 10. Wastewater, 11. Waste oil

DETAILED DESCRIPTION OF THE INVENTION

A first step of the present invention is a step of subjectingmethyl-tert-butyl ether to a decomposition reaction in the presence of asolid acid catalyst to obtain isobutylene, methanol and a dialkyletherand heavy components containing an isobutylene dimer and an isobutylenetrimer. Conditions and specific examples of the first step include thefollowings.

In the first step, a solid acid catalyst, for example, analumina-modified silica is usually used. A gas phase reaction of a fixedbed is usually adopted in the first step, the reaction temperature isusually from 150 to 300° C. and the reaction pressure is usually from anordinary pressure to 10 kg/cm²G. A feed rate of a raw material isselected depending on the reaction temperature, reaction pressure,conversion of desired methyl-tert-butylether or the like, and usuallyfrom 3 to 20 (h⁻¹) in terms of liquid hourly space velocity (LHSV). Thereaction gas is liquefied after cooled in a heat recovery equipment.

In a second step, the reaction liquid obtained in the first step iswashed with water and then separated into an oil layer and a waterlayer. Conditions and specific examples of the second step include thefollowings.

In the second step, any types of extraction column such as a sieve traytype tower and a rotary disk type column can be used. The operationpressure is usually from 6 to 8 kg/cm²G since the reaction liquid istreated in liquid phase. The weight ratio of the washing water/oil isusually from about 0.4 to 0.8, preferably from 0.4 to 0.6 (ratio ofmethanol/water is 1 : 1.5. In addition, a part or whole of waterwithdrawn from the bottom of the column in a fourth step is recovered tobe reused as the washing water.

In a third step, the oil layer obtained in the second step is subjectedto distillation to obtain a fraction containing isobutylene and adialkylether from the top of a distillation column and a fractioncontaining methyl-tert-butyl ether and heavy components from the bottomof the distillation column. Conditions and specific examples of thethird step include the followings.

A usual distillation column such as a sieve tray type column, a valvetray type column or a packed column can be used in the third step. Theoperation pressure is a pressure enough to condense a fraction from thetop of the distillation column at a temperature of a cooling watergenerally used, and preferably from 4 to 6 kg/cm²G.

In the fourth step, the water layer obtained in the second step issubjected to distillation to obtain a fraction containing methanol fromthe top of a distillation column, a fraction containing water from thebottom of the distillation column and a fraction rich in tert -butanolfrom a side cut of the distillation column, and at least a part of thefraction obtained from the side cut is recycled to the first step.Conditions and specific examples of the present step include thefollowings.

A usual distillation column such as a sieve tray type column, a valvetray type column and a packed column can be used in the fourth step.

An operation pressure is usually from an atmospheric pressure to 0.3kg/cm²G. The side cut is a plate in which a concentration oftert-butanol is the highest and usually selected from any oftwenty-eighth to thirtieth plate in theoretical plate number from thetop of the distillation column. The amount of extracted from the sidecut is adjusted so that the amount of tert-butanol contained in the sidecut fraction becomes equal to that of tert-butanol fed into the fourthstep. In addition, the ratio of methanol/tert-butanol/water contained inthe side cut fraction is usually about 80:about 15:about 5, and thewhole or a part of the fraction obtained from the side cut may berecycled to the first step.

The most significant characteristic of the process of the presentinvention is to obtain the fraction rich in tert-butanol from the sidecut of the distillation column and to recycle at least a part of thefraction obtained from the side cut to the first step. By adopting theprocess of the present invention, a methanol product that issubstantially free from water and tert-butanol, is obtained from the topof the distillation column, a loss of methanol contained in the side cutfraction is prevented, and the amount of isobutylene obtained by thedecomposition of tert-butanol can be increased.

In other words, in the case where the side cut is not taken out,tert-butanol is accumulated in the distillation column and thendistilled, and the accumulation of tert-butanol has a function ofbringing water to be distilled from the top of the distillation column,thereby increasing water content in methanol.

EXAMPLE Example 1 and Comparative Example 1

As shown in a flow of FIG. 1, the following first to fourth steps wereemployed.

But, in Comparative Example 1, a withdrawal of a fraction rich intert-butanol from a side cut fraction was not conducted in the fourthstep. Results in Comparative Example 1 are shown in the followingtables. Numerical values in the tables mean wt %.

Abbreviations in the tables are as follows:

MTBE: methyl-tert-butyl ether;

DIB: isobutylene dimer;

DME: dialkyl ether; and

TBA: tert-butanol.

First Step: Methyl-tert-butyl ether was subjected to a decompositionreaction in the presence of an aluminum-containing solid acid catalystto obtain isobutylene, methanol, by-product dialkylether as well asheavy gravity components including an isobutylene dimer and anisobutylene trimer. A fixed bed gaseous phase reaction was employed. Thereaction temperature was 220° C., and the reaction pressure was 6kg/cm²G. The feed rate of the raw material (feedstock) was 3 (h⁻¹) atLHSV base. The reaction gas was liquefied after cooled by a heatrecovery equipment.

Second Step: The reaction liquid obtained by the first step was washedwith water to be separated into an oil layer and a water layer. A sievetray type column was employed for the washing, wherein an operationpressure was 7 kg/cm²G at a top of the tower and a weight ratio of thewashing water/oil was 0.45.

Third Step: The oil layer obtained by the second step was subjected to adistillation to obtain a fraction consisting of isobutylene anddialkylether from a top of a distillation column as well as a fractionconsisting of methyl-tert-butyl ether and heavy components from a bottomof the distillation column. A valve tray type column was used for thedistillation, wherein an operation pressure was 5 kg/cm²G.

Fourth Step: The water layer obtained by the second step was subjectedto a distillation to obtain a fraction consisting of methanol from a topof a distillation column as well as a fraction consisting of water froma bottom of the distillation column. Whole of the fraction obtained fromthe side cut was recycled to the first step. A valve tray type columnwas employed for the distillation, wherein an operation pressure was0.25 kg/cm²G.

TABLE 1 First Step Reaction Feed Effluent MTBE 95 4.3 DIB 0.7 0.9 Water2.9 3.1 Isobutylene 0.0 57.9 Methanol 0.1 32.8 DME 0.0 0.2 TBA 0.8 0.2

TABLE 2 Second Step Oil layer after Water layer after washing washingMTBE 6.3 0.4 DIB 1.5 0.0 Water 0.1 60.0 Isobutylene 90.9 0.8 Methanol0.0 38.6 DME 0.3 0.0 TBA 0.0 0.3

TABLE 3 Third Step Effluent from Effluent from top of column bottom ofcolumn MTBE 0.0 73.0 DIB 0.0 17.3 Water 0.1 0.0 Isobutylene 99.5 0.1Methanol 0.0 0.0 DME 0.3 0.0 TBA 0.0 0.0

TABLE 4 Fourth Step Effluent from Effluent from top of column bottom ofcolumn MTBE 1.0 0.0 DIB 0.0 0.0 Water 0.1 99.7 Isobutylene 1.9 0.0Methanol 96.9 0.3 DME 0.0 0.0 TBA 0.7 0.0

Example 1 was conducted according to the first to fourth steps mentionedabove, namely, was conducted in the same manner as in the ComparativeExample 1 except for conducting the withdrawal of the fraction rich intert-butanol from the side cut fraction in the fourth step.

The whole of the fraction was recycled to the first step. A watercontent in methanol as product was 500 wt ppm or less, and atert-butanol content was 100 wt ppm or less. A loss of methanol wasprevented by 5%, and an amount of isobutylene obtained by tert-butanolrecycled to the first step was improved by 0.3%.

As described above, the present invention provides a process forproducing isobutylene and methanol by decomposing methyl-tert-butylether and separating isobutylene and methanol frommethyl-tert-butylether to recover isobutylene and methanol, whereincontamination of methanol, which is a target product, with water andtert-butanol is suppressed as well as losses of isobutylene and methanolare prevented.

What is claimed is:
 1. A process for producing isobutylene and methanolcomprising decomposing methyl-tert-butyl ether into isobutylene andmethanol to obtain a mixture containing isobutylene and methanol,separating isobutylene and methanol respectively therefrom, therebyindividually recovering isobutylene and methanol, the process comprisingthe following steps: a first step of subjecting methyl-tert-butyl etherto decomposition in the presence of a solid acid catalyst to obtain areaction liquid containing isobutylene, methanol and, as by-products, adialkyl ether and heavy components containing an isobutylene dimer andan isobutylene trimer; a second step of washing the reaction liquidobtained in the first step with water to separate the reaction liquidinto an oil layer and a water layer; a third step of subjecting the oillayer obtained in the second step to distillation using a distillationcolumn to obtain a fraction containing isobutylene and the dialkyletherfrom the top of the distillation column and a fraction containingmethyl-tert-butyl ether and the heavy components from the bottom of thedistillation column; and a fourth step of subjecting the water layerobtained in the second step to distillation using a distillation columnto obtain a fraction containing methanol from the top of thedistillation column, a fraction containing water from the bottom of thedistillation column and a fraction rich in tert-butanol from a side cutof the distillation column, and then recycling at least a part of thefraction obtained from the side cut to the first step.
 2. The processaccording to claim 1, wherein the amount withdrawn from the side cut isadjusted so that the amount of tert-butanol contained in the side cutfraction becomes equal to that of tert-butanol fed into the fourth step.3. The process according to claim 1, wherein the fraction from the sidecut is withdrawn from any of a 28th to 30th plates in theoretical platenumber from the top of the column.